This application claims the priority of European Patent application 00 120 607.7, filed Sep. 21, 2000, the disclosure of which is incorporated herein by reference in its entirety.
Power chucks are used, amongst others, on fully automatic machine tools for manufacturing large series of workpieces machined with very high precision. Because of the high maintenance costs of chucks, they should have a long tool life between successive servicing operations, even when activated and released at a very high frequency for machining successive workpieces, and although they must generally work in an atmosphere full of abrasive dust generated by the grinding of hard steel, which quickly wears out all mechanisms exposed to it. It is known to protect the mechanism which activates and releases the jaws of a chuck from premature wear by enclosing it in a hermetically closed space inside the power chuck and more or less filled with oil. This noticeably reduces the wear of said mechanismxe2x80x94which is typically activated by hydraulic meansxe2x80x94and thus greatly prolongs the interval between successive servicing of the chuck. Such an extension of the servicing interval can be crucial where a mass production is concerned.
However, this static greasing through an enclosed amount of oil has drawbacks. The oil supply must be topped up or replaced at regular intervals, which requires a periodic servicing and is costly on large automatic machine tools, the chuck of which is generally difficult to access. Further, it is usually impossible to maintain a given excess pressure with respect to the atmosphere in the space in which the mechanism to be protected is enclosed, because this space is necessarily sealed off by moving parts. Now such an excess pressure would be necessary in order to completely prevent the intake of foreign matter, in particular of grinding dust.
Hence, it is a general object of the invention to remedy the above-mentioned drawbacks.
In order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the invention is defined as recited in the claims.